1. Field of the Invention
The invention concerns a method of manufacturing formal metal pieces by casting and sintering of a metal alloy powder. The expression "formal" means that the pieces that are obtained by the method are within the shapes and measurements as desired and do not have to be subjected later to a shaping treatment by mechanical distortion. The expression "metal alloy powder" is indicative of the fact that the implemented powder is a powder made of alloy grains, the composition of which is not significantly altered by the execution of the method.
The method of the invention is applied especially to the execution of pieces made of superalloys with a cobalt and/or nickel base or still made of titanium-based alloys. It is of the type that includes first a conforming phase which includes the following operations: introduction of a load of metal alloy powder into a formal casting, heating of the casting under temperature and time conditions that enable obtaining a solid but porous element (which will be called "preform"), and secondly a compacting and sintering phase during which the preform is subjected to a thermal treatment under isostatic pressure and temperature, length and pressure conditions that enable obtaining a compact piece, entirely or mostly without porousness.
2. Description of the Prior Art
Methods such as this one are already familiar. Theoretically, it is possible to substitute one for another advantageously, for obtaining complex shaped metal pieces made of superalloys or of titanium alloys, to the methods that implement tooling inside the mass or isothermic forging at a superplastic phase because they do not display the disadvantages such as significant losses of matter, or further still the number and length of the operations, cost and complexity of the instruments, etc.
An application example of the previously mentioned method is briefly described in the publication by Louis J. Fiedler entitled "Advancements In Superalloy Powder Production And Consolidation" which appeared in "Agard Conference Proceedings No. 200, April 1976", pages 4B-1 to 4B-9. This example concerns the execution of nickel superalloy pieces. The conforming phase is achieved inside a formal rigid casting, heated at 1246 degrees C. in such a way that the executed preform is porous but its pores are closed. Otherwise, the pores do not communicate amongst themselves and do not lead to formation of a passage leading to the outside. During the compacting and sintering phase, the preform is subjected directly to isostatic pressure. This method actually displays two disadvantages, both produced by the fact that the pores must be closed, lest the isostatic pressure can not be applied directly to the preform.
On the one hand, in order to assure that the pores are closed, the heating temperature during the conforming phase must reach a value such that a liquid phase must appear inside the contact zones of the grains. However, if this temperature becomes too high, the ratio of melted and reconsolidated alloys becomes too high, resistance to distortion by compression of the preform becomes too high and isostatic pressure is inadequate. The range of satisfactory temperatures is therefore extremely narrow and difficult to heed.
On the other hand, densifying during this conforming phase reaches a significant value and triggers a shrinkage, the value of which is extremely close to the overall shrinkage as a result of the two phases. In other words, during the conforming phase most of the shrinkage takes place. In some parts of the preform and especially in the concave parts, the preform does not adopt the shapes of the casting anymore. Shrinkage cracks also appear. It therefore is not possible to obtain healthy pieces that have both a complex shape and precise measurements.